1. Field of the Invention
The present invention relates to a method and apparatus for size measurement of particles.
2. Description of the Related Art
Determining the number and size of particles suspended in a liquid solution is important for many industries. Medical applications, such as blood cell counters, have evolved from Coulter's seminal invention, U.S. Pat. No. 2,656,508 issued on Oct. 20, 1953. The preferred embodiment shown in FIGS. 1 and 6 of Coulter's patent was the beginning of what are today generically referred to as “electrical sensing zone” (ESZ) devices or methods. In an ESZ device a particle is measured by passing it through an electrical current-carrying aperture in an insulating partition between two containers, holding a conductive liquid. The motion of the particle through the aperture is caused by a pressure difference across the partition produced by a vacuum or pressure source. The presence of a particle in the aperture increases the electrical resistance of the aperture by displacing the liquid of equal volume to the particle volume. The physical chemistry of particles immersed in an electrolytic solution causes all particles (even those that are electrically conducting) to behave as if they were electrical insulators. The change in resistance may be detected as an increase in voltage across the aperture, or a decrease in current through the aperture. The change in resistance is approximately proportional to the volume of the particle. It is this approximate relationship between aperture resistance change and particle volume that gives the ESZ method good accuracy compared to several other methods of particle size measurement. On the other hand, the ESZ method has comparatively low sensitivity to submicrometer particles due to a multiplicity of noise sources in the aperture, comprising thermal noise due to aperture resistance, noise due to turbulence caused by electrical heating of the liquid in the aperture, noise due to hydrodynamic turbulence caused by flow of liquid through the aperture, noise caused by acoustic interference, noise caused by electrical interference and noise caused by mechanical vibrations. The deleterious effects of noise in the ESZ instrument are magnified by the fact that a signal pulse created by the passing of a particle through the aperture must exceed a predetermined threshold voltage in order to be detected. The threshold voltage must be set high enough so that the peaks of the noise will not falsely trigger the threshold circuit enough times to substantially interfere with the true pulses generated by the particles. The peak of the random noise is several times the root mean square (r.m.s.) level, and this requires the threshold voltage to be set a multiple number of times higher than the r.m.s. noise level. This causes a loss in sensitivity.
The usual aperture shape used in commercial prior art ESZ instruments is the circular cylinder. Until now, this has been considered to be the optimum aperture shape for the highest sensitivity. The circular, cylindrical, aperture shape causes pulses due to particles to have a wide variety of shapes. A pulse due to a particle passing along the axis of the aperture has a single peak. Other particle paths can have pulses with dual peaks, because at both the entrance and the exit of the aperture, the electric field intensity is higher at the edges than at the center. The variations in pulse shapes make it necessary to have a wide bandwidth pulse amplifier so that the pulses would be reproduced by the amplifier with high fidelity. The wide bandwidth increases the noise level. The thickness of the insulating partition through which the aperture is bored must be at least 30 micrometers (μm), the length-to-diameter ratio of the aperture is large, and this results in poor sensitivity. This is because the internal volume of the aperture must be small for high sensitivity, and a large thickness of the partition makes the aperture length long, which makes the aperture volume large.
The circular, cylindrical, aperture causes boundary-layer separation of the liquid therein. The boundary-layer separation produces noise due to turbulence. At the input of the aperture, there is a hydrodynamic constriction of the fluid flow diameter. At the output of the aperture, there is a jet oriented along the axis that is surrounded by backflow of toroidal shape. Some of the particles that have passed through the aperture are captured by the backflow and recirculate near the exit of the aperture, and this causes noise in the form of false particle pulses.
The measurement range, or “dynamic range” of an aperture is limited to about 20/1 in particle diameter, making the ESZ instrument useful for relatively narrow particle size distributions.
Most particle size analysis done with the ESZ instrument is done with apertures 30 μm in diameter, or larger. Apertures smaller in diameter have been available commercially for prior art instruments, but these smaller apertures have been little used because of their tendency to be plugged by debris. If the aperture is plugged, the instrument cannot continue to measure particles until the plug is dislodged.
Even if debris does not cause plugging, it can cause particles to be measured that are not part of the sample being tested. To keep debris from interfering with the sample, the sample concentration must be significantly higher than the debris concentration. But the sample concentration must be low enough to avoid multiple sample particles from being simultaneously in the aperture. This is called “coincidence.” Coincidence of particles in the aperture can cause errors in the measured particle size distribution.
U.S. Pat. No. 3,742,348 issued on Jun. 26, 1973 to Golibersuch, addresses the problems related to plugging by debris, coincidence, and low sensitivity to small particles, by allowing a multitude of particles to pass though the aperture simultaneously. This allows a larger diameter aperture to be used than is needed to detect single particles, thus reducing the frequency of plugging. Also, smaller particles can be detected than by a single-particle measurement. One of the disadvantages of this invention is that the circular, cylindrical, aperture requires a large length-to-diameter ratio (on the order of 20/1) in order to get a signal that can be analyzed by the signal processor proposed by the inventor. The only information it obtains is equivalent to what I call “λ2” in a subsequent section of this specification, for the special case where all of the particles in the sample have equal volumes. It will be apparent that μ2, by itself, is not a significant amount of information about the particle size distribution. Also, assuming all of the particles to be the same size is a very unrealistic and restrictive assumption.
Applicant believes that the closest reference corresponds to U.S. Pat. No. 4,926,114 issued to Doutre on May 15, 1990 for an apparatus for studying particles having an aperture whose cross-sectional area changes progressively along at least part of its length. Doutre's patent discloses an apparatus and method for studying particles suspended in molten metal that includes an aperture with a current path therethrough, causing the fluid to flow through the aperture and detecting resistive pulses caused by the passage of suspended particles. An aperture with a cross-section that changes progressively along its length provides additional information about particle size. However, it differs from the present invention because it has the same sensitivity limitations of the other prior art devices since it does not keep the Reynold's number of the fluid flow within the required range to minimize or preclude boundary layer separation effects. Doutre's patent is not even concerned with these effects since it applies to molten metals. In fact, Doutre's invention is intended for use with apertures that ideally contain no more than one particle at a time. Col. 4, lines 51-53. There is not even a suggestion of using statistical moments to make inferences about the size of the particles.
Other patents describing the closest subject matter provide for a number of more or less complicated features that fail to solve the problem in an efficient and economical way. None of these patents suggest the novel features of the present invention.